1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for forming a micro-lens by an ion injection process without a reflow process, so as to obtain a desired refractive index distribution, thereby accurately controlling light concentration.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device which converts an optical image to an electric signal. There are two kinds of image sensor devices: a charge coupled device (hereinafter referred to as CCD) and a complementary metal-oxide semiconductor (hereinafter referred to as CMOS). The CMOS image sensor includes a photodiode unit which detects irradiated light, and a CMOS logic circuit which processes the electric signal to form data. Moreover, as the amount of light received in the photodiode increases, the photo-sensitivity of the light sensor will increase accordingly.
In order to increase the photo sensitivity, either a fill factor of the photodiode area within the total area of the image sensor needs to be increased, or an optical path of the light irradiated process to the area outside of the photodiode needs to be changed so as to focus light onto the photodiode. The main examples of the light-focusing method would be to form a micro-lens. The related art shows a convex micro-lens formed of a material having an excellent light-transmission ratio is provided on the photodiode. Thus, the optical path of the irradiated light is refracted, so as to irradiate a larger amount of light to the photodiode. In this case, the micro-lens refracts the light being parallel to the optical axis, so as to form a focal point of the light on a specific point of the optical path.
A related art CMOS image sensor and fabrication of a micro-lens thereof will now be described with reference to the accompanying drawings. FIG. 1 is a cross-sectional view of a related art CMOS image sensor. As shown in FIG. 1, the related art CMOS image sensor includes a semiconductor substrate (not shown), a sub layer 11, an interlayer dielectric 12, R, G, and B color filter layers 13, a planarization layer 14, and a micro-lens 15. The sub layer 11 includes at least one photodiode area and metal lines, wherein the photodiode area generates electric charges depending upon the amount of irradiated light. The interlayer dielectric 12 is formed on an upper surface of the sub layer. R, G, and B color filter layers 13 formed on the insulation layer 12 enable light of each specific wavelength to pass through. The planarization layer 14 is formed on the R, G, and B color filter layers 13. And, the micro-lens 15 is formed on the planarization layer 14 and has a convex shape having a set curvature, so that light can pass through the color filter layers 13 facing thereto and that the light is focused onto the photodiode area.
Furthermore, although not shown in the drawings, an optical shielding layer is formed within the interlayer dielectric. The optical shielding layer is formed to prevent light from being irradiated onto an area outside of the photodiode area. Also, the photosensitive device may also be formed to have a structure of a photo-gate, instead of the structure of a photodiode.
Herein, the micro-lens 15 is mainly formed of a polymer group resin by using deposition, patterning, and reflow processes. Additionally, a curvature and thickness of the micro-lens 15 are decided by taking into account various factors, such as the focal point of a focused light. More specifically, the micro-lens 15 should be formed to have optimum size, thickness, and curvature radius, which is decided by the size, position, shape of the unit pixel, the thickness of the photosensitive device, and the height, position, and size of the optical shielding layer.
The most important process in the fabrication process of the micro-lens is a space CD controlling process, in which the space between the lenses is controlled, and also a curvature forming process, in which the optimum curvature of the lens is formed. More specifically, by adequately controlling the curvature of the lens, the optical signal passing through a color filter array is efficiently transmitted to the photosensitive device.
However, the related art CMOS image sensor and method of fabricating same have the following disadvantages. In the related art fabrication method, the CMOS image sensor is fabricated by first forming a rectangular lens array, which is spaced apart by using photoresist layers, and then forming the curvature of the lens by using a reflow process. Thus, it is difficult to provide an adequate space between the lenses. Moreover, when the lenses are not accurately spaced apart from one another, excessive noise may occur during an image processing step, or a sufficient size of the signal may not be provided.
Furthermore, when using the above-described semi-spherical micro-lens, the irradiated light, which is parallel to the optical axis, is refracted from the lens, so as to be transmitted to the photosensitive device facing into the lens, thereby driving the device. However, when light that is not parallel to the optical axis is refracted from the lens, the refracted light may be incorrectly irradiated to a photosensitive device located in a different optical path, thereby causing misoperation. And, finally, the difference in the amount of light being transmitted to the photosensitive device may vary depending upon the type of material and size used to form the lower layer of the micro-lens, thereby reducing light-focusing efficiency and deteriorating image quality.